Microorganisms pervade our environment affecting our lives in both beneficial and harmful ways. For this reason there is an ever increasing requirement to provide a sensitive, effective, speedy mechanism for the detection, identification and study of the presence and metabolic activity of microorganisms.
In recent years the science of microbiology has experienced considerable advancements. This is particularly true for the field of sensors used in the detection, identification and analysis of the behavior of microorganisms. While progress has been made in the field of monitoring microorganisms, inefficiencies in the commonly used monitoring methods still exist. For example, a slow but effective procedure for antimicrobic susceptibility testing, the Bauer-Kirby Disc Method, is still used in hospital environments. This method uses the presence or absence of visible growth of the microorganisms to indicate the efficacy of an antimicrobic compound, and generally requires an 18 to 24 hour incubation period to allow for microorganism growth before a result can be obtained.
Another popular method for antimicrobic susceptibility testing is the broth micro-dilution method, such as the Sceptor RTM. System for identification and antimicrobic susceptibility testing of organisms (Becton Dickinson Diagnostic Instrumentation Systems, Sparks, Md.). That system uses a disposable plastic panel having a plurality of low volume cupulas (ca. 0.4 ml per cupula), each containing a different test compound or a different concentration of a test compound dried on the cupula surface. The organism to be tested is suspended in the desired testing medium, and aliquots are delivered to the individual cupulas of the test panel. The reagent dried on the panel dissolves in the sample, and the system is then incubated overnight (18 to 24 hrs.) to allow sufficient time for the organisms to interact with the reagent and for visible growth to appear. The panel is subsequently examined visually for the presence or absence of growth, thereby obtaining information on the susceptibility of the organism undergoing testing. Additional wells aid in identifying the organism. However, this test method, like the Bauer-Kirby Disc Method, suffers from the drawback of also requiring a long incubation period.
In recent years efforts have been made to avoid the long incubation times required for the above discussed monitoring methods. These innovations have focused on the monitoring of metabolic activity of microorganisms rather than monitoring the growth of colonies. Many approaches to monitoring metabolic activity of microorganisms have been reported in the attempt to rapidly and accurately monitor such metabolic activity.
One innovation in the field of microorganism monitoring is an apparatus, which utilizes light scattering optical means to determine susceptibility by probing the change in size or number of microorganisms in the presence of various antimicrobic compounds. An example of commercial instruments, which utilize this methodology is embodied by the Vitec System (BioMerieux Corp.). At best this system is expected to yield information on antimicrobic susceptibility of microorganisms within 6 hours for many organism and drug combinations. Other combinations can require as long as 18 hours before the antimicrobic susceptibility of the organism can be determined by the Vitech System method.
In an effort to improve on the Bauer-Kirby procedure, modifications have been developed which allow certain samples to be read in four to six hours. However, this modified system is destructive in nature, requiring the spraying of a developing solution of a color forming dye onto the test plate. The destructive effect of the developing solution prohibits re-incubation and reading at a later time if the initial rapid technique fails. Thus, the experiment cannot be continued for a standard evaluation at a later time.
Still other approaches have involved monitoring of microbial oxygen consumption by the measurement of pH and/or hemoglobin color change, or by the use of dyes such as triphenyl-tetrazolium chloride and resazurin, that change color in response to the total redox potential of the liquid test medium.
The monitoring of the consumption of dissolved oxygen by microorganisms, as a marker of their metabolism, has been studied for many years. For example, C. E. Clifton monitored the oxygen consumption of microorganisms over a period of several days using a Warburg flask in 1937. This method measured the change in oxygen concentration in a slow and cumbersome manner.
The “Clark” electrode, an electrochemical device, is also commonly used to measure dissolved oxygen. Unfortunately, the Clark electrode consumes oxygen during use (thereby reducing the oxygen available to the microorganisms) and the “standard” size electrode is typically used only to measure volumes of 100 mls or greater to prevent the electrode from interfering with the measurements.
A “miniature” Clark electrode has been described, but this electrode is a complicated multi-component part, which, like the larger electrode, must be in contact with the solution being measured. While an oxygen permeable membrane can be used to prevent the electrode components of the device from interacting with the constituents of the test solution, the oxygen must still equilibrate between the test solution and the measurement system and is consumed once it passes the membrane.
Optical systems which can yield oxygen concentration data, have been developed to overcome the shortcomings of the Clark electrode systems. The main advantage of such optical methods is that the instrumentation required to determine quantitative value does not itself make physical contact with the test solution. Optical techniques allowing both calorimetric and fluorometric analyses for oxygen to be carried out rapidly and reproducibly are known, and costs for such analyses are often quite low. For example, several luminescent techniques for the determination of oxygen have been described which are based on the ability of oxygen to quench the fluorescence or phosphorescence emissions of a variety of compounds. However, such methods have not been readily adapted to microbial monitoring. Further, such systems, like the Clarke Electrode system are limited to monitoring only the consumption of dissolved oxygen by microorganisms.
Other systems have been described that provide information on the presence, identity and antimicrobic susceptibility of microorganisms in a period of eight hours or less. Wilkins and Stones in U.S. Pat. No. 4,200,493 disclose a system that uses electrodes and a high impedance potentiometer to determine the presence of microorganisms. In U.S. Pat. No. 3,907,646 Wilkins et al disclose an analytical method which utilizes the pressure changes in the headspace over a flask associated with microbial growth for the detection and surveillance of the organisms. U.S. Pat. No. 4,220,715 to Ahnell, discloses a system wherein the head space gas above a test sample is passed through an external oxygen detector for determination of the presence of microorganisms. Ahnell, in U.S. Pat. No. 4,152,213, discloses a system for analysis by monitoring the vacuum produced by growing organisms in a closed head space above a test sample. U.S. Pat. No. 4,116,775 to Charles et. al is an example of the use of optical means based on the increase in turbidity or optical density of a growing microbial culture for the detection and monitoring of bacterial growth. As with the Clarke Electrode systems, these systems are designed to provide data limited to oxygen consumption.
U.S. Pat. No. 5,629,533 issued to Ackley et al. is exemplary of optical sensors, which have been developed for monitoring carbon dioxide levels on a continuous basis. Such sensors involve the use of glass fiber optics in combination with a sol-gel sensor element, which contains a chemical indicator sensitive to the presence of carbon dioxide. This system encompasses a grooved substrate with a sol-gel material having a chemical indicator adhered within the grooves. Fiber optic cables are coupled to the grooves and as light is passed through the fiber optic cables, the transmission is affected by the sol-gel sensor element.
While testing methods as exemplified above have improved in recent years to provide faster more accurate means of detecting the growth and metabolic activity of microorganisms, it is a common shortcoming of such testing methods that none of the innovations have provided a biological sensor that is capable of simultaneously detecting growth of both anerobic and aerobic microorganisms in a sample. This limitation in commonly used monitoring systems exists because such systems typically allow only one gas component to be detected in one sensor unit.
Gas composition monitoring systems, which can be used to detect metabolic activity of microorganisms are limited to monitoring either oxygen or carbon dioxide. Carbon dioxide sensors can use an acid-base indicator chromophore to modulate the signal output of a fluorophore. In such a system the chromophore absorbance spectrum changes when the pH value of a measured sample changes. The system is able to determine the carbon dioxide level because the pH value of a measured sample depends upon the carbon dioxide level of the sample environment. A monitoring system, which determines metabolic activity of microorgansims by employing an oxygen sensor, can employ an oxygen sensitive fluorophore to detect the oxygen level changes in a sample environment. Prior to the present invention, the combination of the ability to monitor both carbon dioxide and oxygen levels in a gas composition simultaneously was frustrated by the overwhelming problem of cross-talk between the system sensors.
The present invention addresses this problem by providing a sensor formulation and system that can respond independently and simultaneously to oxygen and carbon dioxide.